This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application No. 2000-298110 filed on Sep. 29, 2000, the entire content of which is incorporated herein by reference.
This invention generally relates to a vehicle motion control system. More particularly, the present invention pertains to a vehicle motion control system which supplies hydraulic pressure generated by an automatic hydraulic pressure generator, including irrespective of a brake pedal operation, to a wheel brake cylinder via a control valve. This vehicle motion control system is applicable to a brake steering controller and to a traction controller.
A known type of vehicle motion control system is described in Japanese Patent Laid-Open Publication No. 5-213172 published in 1993. This vehicle motion control system uses a vacuum booster to obtain sufficient braking force for vehicle motion control. In this vehicle motion control system, when the system is unable to generate sufficient braking force during slip control, the system prohibits the starting of braking operation. More specifically, it is judged whether or not a vacuum pressure in the constant pressure chamber of the vacuum booster is sufficient under non-braking condition from a vacuum condition detecting source. If the vacuum pressure in the constant pressure chamber does not have the required vacuum pressure, the brake control is prohibited. The system uses engine intake as a vacuum source and vacuum pressure is computed by using a throttle opening and a engine rotation sensor.
The known system described above suffers from several drawbacks. According to the known slip control system, when sufficient vacuum pressure is not maintained in the constant pressure chamber, braking control is prohibited by a control starting restriction means. According to this control, the system is not capable of enabling precise braking control to perform appropriate vehicle motion control. To perform precise vehicle motion control, it is necessary to adjust the starting point of the vacuum booster or control the variance of the braking force during the vacuum booster operation.
A need thus exists for a vehicle motion control system that adjusts the starting point of the vacuum booster operation to perform a more precise vehicle motion control even if the constant pressure chamber of the vacuum booster does not have sufficient vacuum pressure.
A need also exists for a vehicle motion control system that is able to adjust or vary the braking force in accordance with a decline in the vacuum pressure in the constant pressure chamber to control the vehicle motion even if the constant pressure chamber of the vacuum booster does not have sufficient vacuum pressure.
According to one aspect of the invention, a vehicle motion control system includes a wheel brake cylinder for applying a braking force to a wheel, an automatic hydraulic pressure generator which generates a hydraulic braking pressure by driving a vacuum booster irrespective of brake pedal operation, a hydraulic pressure control valve disposed between the automatic hydraulic pressure generator and the wheel brake cylinder to adjust hydraulic brake pressure supplied to the wheel brake cylinder, a brake controller for performing vehicle motion control by controlling the automatic hydraulic pressure generator and the hydraulic pressure control valve in accordance with a vehicle motion condition irrespective of brake pedal operation, and a vacuum pressure detecting means for detecting a vacuum pressure of the vacuum booster. The brake controller controls the starting of vehicle motion control in the direction that the vehicle motion control is restricted when the vacuum pressure which drives the vacuum booster is relatively small.
According to another aspect of the invention, a vehicle motion control system includes a wheel brake cylinder for applying a braking force to a wheel, an automatic hydraulic pressure generator which generates a hydraulic braking pressure by driving a vacuum booster irrespective of brake pedal operation, a hydraulic pressure control valve disposed between the automatic hydraulic pressure generator and the wheel brake cylinder to adjust hydraulic brake pressure supplied to the wheel brake cylinder, a brake controller for performing vehicle motion control by controlling the automatic hydraulic pressure generator and the hydraulic pressure control valve in accordance with a vehicle motion condition irrespective of brake pedal operation, and a vacuum pressure detector for detecting a vacuum pressure of the vacuum booster. The brake controller controls the duration of the hydraulic brake pressure increase in the wheel cylinder by the hydraulic pressure control valve in the direction to be longer when the vacuum pressure which drives said vacuum booster is relatively small.